Ultracapacitors, also known as double layer capacitors, DLCs, supercapacitors or psuedocapacitors are devices that store electrical energy. Ultracapacitor are increasingly being used to power consumer products, automotive energy storage systems, military applications, etc. as the sole energy storage device or they in combination with batteries.
After their charge is depleted, ultracapacitors are recharged. Ultracapacitors have to go through a fairly large voltage swing to be used as an energy storage device and must be charged carefully to prevent damage. Because ultracapacitors are sensitive to being charged over their rated voltage, overcharging can result in significantly reduced life or failure.
Another issue in recharging ultracapacitors is in capacitance variance. The capacitance of an ultracapacitor will vary from its rated value, usually by no more than ±20%. Therefore, a series connected string of ultracapacitors will likely have cells with different capacitances. When a series connected string is charged, the voltages of the cells will become different from one another because cells with smaller capacitances will charge more rapidly than cells with larger capacitances. This is apparent from Equation 1 (below) which relates the current, voltage, and capacitance of an ideal capacitor.
                                          i            c                    ⁡                      (            t            )                          =                  C          ⁢                                    ⅆ                              V                c                                                    ⅆ              t                                                          (        1        )            
Charged ultracapacitors also experience leakage or self-discharge. This is where energy is internally dissipated thereby reducing the ultracapacitor's voltage. All ultracapacitors do not self-discharge at the same rate. Due to capacitance tolerances and varying leakage, series connected ultracapacitors will often have voltages different from one another.
Current ultracapacitor charging technology uses balancing circuits to try to make every cell in an ultracapacitor array have equal voltage while the ultracapacitor cells are being charged by a power source. In other words, the current method “balances” t he cells. This is done or could be done in five ways.
First, an active circuit is placed over two series connected cells. The circuit compares the voltage of the two cells and then dissipates the energy in the cell that has the highest voltage. Another balancing circuit is then used to balance one of those two cells with the next cell in the series string. Circuits can be connected in this way to balance many series connected cells. This type of balancing has the following limitations: (1) because it takes time to balance all of the cells, the charging cannot be done rapidly . . . rapid charging of the cells would not allow enough time to ensure proper balancing and some cells in a series string could become over charged; and (2) balancing circuits consume energy and reduce the voltage of every cell to the lowest voltage cell, which wastes energy.
Second, an active bypass circuit is placed over each cell that dissipates energy from the cell through a resistor when the cell gets close to its maximum rating. However, if the cells are charged at a current rate that is higher than what the circuit can bypass, the cell could become overcharged.
Third, a zener diode having a breakdown voltage close to the rated voltage of the ultracapacitor cell is placed over each cell in a series string. When the cell becomes close to its rated voltage the zener starts to conduct current. This has the same problem as the active bypass circuit of the second way (above). It can only protect the cell if it can bypass as much current as the cell is being charged by. This also wastes energy close to the cells rated voltage because a zener does not have a distinct breakdown voltage.
Fourth, a passive resistor with a small resistive tolerance is placed over each cell. This causes a current to flow, which is significantly higher than the cells leakage current. The higher voltage cells dissipate more energy because more current is drawn through the resistor. Ohm's law, I=V/R, shows why this is true. This method consumes a significant amount of energy from the ultracapacitors and balances the cells very slowly. This slow balancing does not allow the cells to be charged from a low voltage to their maximum voltage rapidly without possibly overcharging one or more of the cells.
Fifth, combinations of the above methods.
The prior art is replete with examples employing such methods. For instance, U.S. Pat. No. 6,664,766 shows a supercapacitor balancing system and method (balancing); U.S. Pat. No. 6,265,851 shows an ultracapacitor power supply for an electric vehicle (bypass); U.S. Pat. No. 6,836,098 shows a battery charging method using supercapacitors at two stages; U.S. Pat. No. 7,042,187 shows a control circuit; and U.S. Pat. No. 6,847,192 shows a power supply for an electrical load.